1. Field of Invention
This invention relates generally to heat generators, and more particularly to a ribbon-type, gas-fired burner head which projects a generally planar, omni-directional flame whose intensity is substantially uniform in all directions, which head when combined with a refractory body forms an infrared radiation heater.
2. Status of Prior Art
The transfer of heat takes place by three processes: conduction, convection and radiation. In conduction, heat is transferred through a body by the short range interaction of molecules and/or electrons. Convection involves the transfer of heat by the combined mechanisms of fluid mixing and conduction. In radiation, electromagnetic energy is emitted toward a body and the energy incident thereto is absorbed by the body to raise its temperature. Radiant heating, therefore, differs from both convection and conduction heating, for the presence of matter is not required for the transmission of radiant energy.
According to the Stefan-Boltzmann law, the rate of heat transfer between a source of radiated heat whose temperature is T.sub.s and an absorbing body whose temperature is T.sub.b is equal to T.sub.s.sup.4 -T.sub.b.sup.4 ; that is, to the difference between the fourth powers of these temperature values. In convection heating, the rate of heat transfer is proportional only to the temperature difference between the body being heated and the surrounding atmosphere. Hence convection heating is inherently very slow, as compared to the nearly instantaneous effects of radiant heating.
An IR heater in accordance with the invention may be used throughout the full range of heating applications, including industrial processes such as industrial finishing and textile treatment, as well as in annealing, curing and drying operations which require heating.
It is known to provide infrared heaters in which a refractory body is heated by means of a ribbon-type burner to an elevated temperature causing it to emit infrared radiation. The ribbon-type burner is of the type disclosed, for example, in the Flynn U.S. Pat. No. 3,437,322, in which a gas-air-fuel mixture is fed into a cylinder having a longitudinal slot therein occupied by a stack of corrugated ribbons to create an array of minute jet openings through which the gas-air mixture is expelled. Because of the myriad of jet openings, the projected flame is not composed of discrete jets but assumes a sheet-like form.
However, the intensity of the flame is not uniform throughout the length of the ribbon, for the pressure of the gas-air mixture in the cylinder is not equalized throughout its length. Hence, the resultant infrared radiation pattern is not of uniform intensity; and when food is subjected to this pattern, the heating thereof may be uneven.
In order to overcome this problem, my prior U.S. Pat. No. 4,507,083 (1985) discloses an infrared heater for projecting an infrared beam in a radiation pattern having a predetermined geometry for irradiating the surface of a food product or other body to effect uniform heating thereof at a rapid rate. The heater includes a ribbon-type, gas-fired burner having an elongated pre-mix casing into which is fed air and gas, and an outlet extending along a slot in the casing and projecting therefrom. The outlet is provided with two sets of corrugated ribbons separated by a gas pressure chamber, whereby the air-gas mixture from the casing passes through one set into the chamber where the pressure thereof is equalized before the mixture passes through the other set from which it emerges as a sheet of flame of uniform intensity. The outlet is inserted in the longitudinal socet of a refractory body to impinge on a surface thereof whereby the surface is heated to a temperature level causing the surface to emit infrared energy which is projected by an array of radiation horns formed in the assembly.
In the ribbon-gas-fired burner disclosed in my prior U.S. Pat. No. 4,507,083 (1985), as well as in my prior U.S. Pat. Nos. 4,432,727 (1984) and 4,702,693 (1987), the burner takes the form of an elongated air-gas mixture chamber having a longitudinally-extending outlet occupied by a stack of corrugated ribbons. The difficulty experienced with this arrangement arises from the fact that one end of the elongated chamber or pipe is closed, the other end communicating with an inlet into which is fed pressurized combustion air and a gaseous fuel.
As a consequence of this arrangement, the pressurized air-gas mixture fed into the open end of the chamber travels through the chamber to impinge on the closed end thereof and is reflected thereby in countercurrent relation to the incoming mixture, thereby creating internal turbulence within the chamber which results in pressure variations along the length of the chamber.
Hence when the mixture emitted from the myriad of jet openings created by the ribbon stack is ignited, the sheet of flame emerging from the jet openings is not of uniform intensity along its length. When this gas-fired burner is combined with a refractory body to generate infrared radiation in a predetermined radiation pattern, the radiation intensity is not uniform throughout the IR radiation pattern; and when this IR heater is used to heat food or other objects irradiated by the heater, the heating is uneven.